The Third Generation Partnership Project (3GPP) organization develops the world-harmonized specifications for mobile, multimedia, wireless cellular communications. There are five specified 3 G modes, three of which are CDMA. The CDMA modes are CDMA2000, wideband CDMA (W-CDMA) and time division synchronous CDMA (TD-SCDMA). The 3 G operating modes W-CDMA and CDMA2000 are by far the most dominant in terms of current commercial services, operator deployment plans and vendor support. W-CDMA is the 3 G system of choice for GSM based networks (world-wide), whereas CDMA2000 is used primarily with the IS-41 based networks (mostly in the United States). The TD-SCDMA mode is primarily associated with China.
The rake receiver is the most popular and conventional CDMA baseband receiver. It is called the rake receiver because it has the structure of a rake. FIG. 1 shows a block diagram of certain processes performed by a system implementing the rake receiver based on a single receive antenna 110. The rake receiver contains a bank of despreaders 120 whose input differ by a number of selected sample delays 140. Each despreader despreads the received chip sequence using the spread code assigned by the network management to the individual user/subscriber. The resulting outputs are combined using a linear combiner 160. In order to get the maximum signal-to-noise ratio (SNR), a maximum ratio combining (MRC) scheme can be used in the linear combiner. The output of the rake receiver is the basis for making a received symbol decision element 180.
The problem with the rake receiver is its poor performance in certain types of multipath propagation channels, especially for multiuser CDMA downlink signals. For example, it is becoming increasingly recognized within the technical community that the performance of the conventional rake receiver is insufficient to support the operation of the 3GPP specified high speed downlink packet access (HSDPA) protocol in one of the specified multipath test channels for HSDPA, i.e., the “120 kilometer per hour Vehicular Channel A”.
For a multipath radio frequency (RF) propagation channel, the higher the 3 G CDMA downlink “loading”, the poorer the rake receiver performs. The term “loading” refers to the number, K, of user data-channels that are simultaneously transmitted in the downlink. To separate the user data-channels, K different orthogonal short spreading codes, such as Walsh-Hadamard codes or the W-CDMA specified orthogonal variable spreading factor (OVSF) codes, are applied to the K streams of user data before summing the coded data to form a “loaded” signal.
A reason the rake receiver performs worse as the CDMA downlink loading is increased can be explained by viewing the rake receiver as a multipath-incorporating matched filter, in contrast to a multipath-correcting filter. For single user CDMA signals, the rake receiver is relatively well motivated. However, in for example a W-CDMA downlink signal that is carrying K>1 OVSF spread data-channels in a multipath RF propagation channel, the multipath destroys the orthogonality of the K OVSF codes as presented to the receiver. The loss of orthogonality of the OVSF codes results in the multiple user data-channels appearing as multiple access interference (MAI) to each other. In the rake receiver, severe MAI results for even a relatively small number of users in a given cell when multipath is present.
As an illustration of the problem, consider a 3 G CDMA system, W-CDMA for example, with a base station that is serving a specific user, say Sam, that happens to be in a difficult multipath propagation environment and is using a W-CDMA handset containing a conventional rake receiver. If Sam is to be provided a reliable connection, the W-CDMA base station must reduce the service to other users sharing Sam's downlink signal and/or reduce the data rate to Sam. Reducing the number of multiple users reduces the MAI in the rake receiver that results from the loss of OVSF orthogonality, thus improving its performance. Reducing the data rate to Sam by increasing the OVSF code length, i.e., the spread factor SF, used to carry the data symbols, typically improves his handset's rake receiver performance by brute force SIR improvement. However, the latter technique may be ineffective due to the continued loss of OVSF orthogonality. In any case, reducing the number of user-data channels in the downlink and/or increasing the SF for Sam results in a reduced capacity for the serving W-CDMA base station. Ultimately, this loss of base station capacity translates to a loss of revenue to the service provider.
Other, more complex, CDMA receiver designs have been proposed in order to improve the performance of user equipment (UE) for the multiuser CDMA downlink signal. For example, one such design uses two chip-level linear equalizers and two antennas. FIG. 2 shows a block diagram of certain processes performed by a CDMA receiver system implementing two chip-level linear equalizers 220 and two receive antennas 210. The output of the equalizers is summed by summing element 240 and despread by dispreading element 260 so that the user data of interest can be decided by decision element 280. The coefficients of chip-level linear equalizers 220 can be obtained from estimates of the channel impulse response (CIR) provided a matrix inversion is performed. The development and performance evaluation of the two antenna/two chip-level linear equalizer, receiver diagramed in FIG. 2 has been published by T. P. Krauss, M. D. Zoltowski and G. Leus: “Simple MMSE Equalizers for CDMA Downlink to Restore Chip Sequence. Comparison to Zero-Forcing and Rake”, Proc. IEEE ICASSP 2000, pp. 2865-2868. If only one antenna/linear equalizer is used the performance improvement relative to the rake receiver is significantly diminished.
Another example of a more complex CDMA receiver design that has been proposed in order to improve the performance for the multiuser CDMA downlink signal is the multiuser multistage interference canceller. FIG. 3 shows a block diagram of certain processes performed by a system implementing a multiuser multistage interference canceller for a CDMA receiver. In this interference cancellation scheme, the signal is first passed through a bank of correlators 320 and then each user's signal is reconstructed and cancelled by subtraction element 340 from the received signal. This process may be repeated for multiple stages. The effectiveness of the cancellation depends on the estimation element 360, which involves symbol decision and signal gain/phase estimation, and the reconstruction element 380, which involves re-spreading. For a general discussion of the multiuser multistage interference canceller of FIG. 3, e.g., as implemented to perform successive interference cancellation or parallel interference cancellation, see George Aliftiras, “Receiver Implementations for a CDMA Cellular System”, Masters Thesis, Dept. of Electrical Engineering, Virginia Polytechnic Institute, July 1996. For a detailed development, complexity analysis and performance evaluation of the multipath interference canceller (MPIC) see K. Higuchi, A. Fujiwara, M. Sawahashi, “Multipath Interference Canceller for High-Speed Packet Transmission with Adaptive Modulation and Coding Scheme in W-CDMA Forward Link” IEEE J. Selected Areas in Communications, Vol. 29, No. 2, February 2002. The MPIC was also presented by NTT DoCoMo in several contributions to the TSG-RAN Working Group 1 of 3GPP.
Another example of a more complex CDMA receiver design that has been proposed in order to improve the UE performance for the multiuser CDMA downlink signal is an iterative receiver with a chip-level decision feedback equalizer (DFE). FIG. 4 shows a block diagram of certain processes performed by a system implementing an iterative chip-level DFE. In this example, the CDMA signal contains orthogonal spread data for K users and is received with L antennas. The despread output of a bank of K rake receivers 410 is used to provide the initial chip-level decisions for the feedback filter 420 of the DFE. This mode is indicated by position I of the feedback switch 415. To generate these initial chip-level decisions, the multiuser despread symbol decisions from decision elements 430 are re-spread by spreading elements 440 and summed by summation element 445. The chip-level DFE output 450 is then despread by despreading elements 460 and symbols are decided by decision elements 470 for each of the K users. These symbol decisions are re-spread by spreading elements 380 and summed by summation element 485 to provide the chip-level decisions for the feedback filter 420 during the subsequent iteration(s) of the DFE. This mode is indicated by position F of the feedback switch 415. The development and performance evaluation of the iterative chip-level DFE diagramed in FIG. 4. has been published by J. Choi, S. Rag Kim, and Cheng-Chew Lim: “Receivers with Chip-Level DFE for CDMA Down/ink Channels”, IEEE Trans. Wireless Comm., No. 1, January 2004. These authors point out that the iterative chip-level DFE provides superior performance to all of the above mentioned CDMA receiver designs in the multipath channels tested. The reason the iterative chip-level DFE is somewhat superior to the MPIC is that the latter is more sensitive to incorrect tentative decisions. However, for even moderately loaded CDMA downlink signals, all of the above mentioned complex receiver designs (i.e., FIGS. 2 to 4) significantly outperform the rake receiver (FIG. 1) in the multipath channels tested.
Another example of a more complex CDMA receiver design that has been proposed in order to improve the performance for the multiuser CDMA downlink signal is the use of any of the above receiver structures to support a multiple input multiple output (MIMO) use of the propagation channel. In a MIMO system, multiple receive antennas are used in conjunction with transmission over multiple transmit antennas using a signal coding scheme, e.g., space time coding (STC), to provide additional diversity gain over that available with conventional multiple receive/transmit antenna diversity. Although a MIMO CDMA system can increase the capacity and reliability of the wireless channel, the receiver complexity is further increased, not only at the level of baseband processing discussed here but also at the levels of the RF receiver/transmitter front-end, the antennas, and radio management overhead. For an explicit example, see the development and performance evaluation of the iterative chip-level DFE for MIMO CDMA by Agus Santoso, “Chip-level DFE for the CDMA Downlink Channel”, Masters Thesis, Dept. of Electrical Engineering, University of Adelaide, Australia, October 2003.